The present invention relates to a polishing composition used to polish an object such as a substrate for a magnetic disk, and a method for polishing an object using such a polishing composition.
A substrate for a magnetic disk is usually manufactured through a plurality of polishing steps. In general, in a first polishing step, a polishing composition containing alumina, an organic acid, and water is used (see, for example, Japanese Laid-Open Patent Publication Nos. 2-84485 and 2003-170349). For the purpose of obtaining a substrate having few surface defects such as pits, it is preferable that the particle size of alumina contained in the polishing composition be as small as possible. However, most polishing compositions containing alumina having small particle sizes are incapable of rapidly polishing a substrate. On the other hand, as a magnetic disk becomes higher in capacity higher flatness is demanded for a substrate for the magnet disk. One of the factors causing degradation in the flatness of a substrate is surface defects; a substrate abundant in defects is poor in flatness.